Conventionally, various vehicle control systems have been proposed with the aim of improving the lateral stability of the vehicle and the driveability of the vehicle. Such vehicle control systems include traction control systems and four wheel steering systems. Such vehicle control systems are intended to optimally control the throttle, the brake, the steering system and other parts of the vehicle according to the dynamic properties of the vehicle, the dynamic conditions of the vehicle, and the operations carried out by the vehicle operator.
When the vehicle operator is a skilled one, and is familiar with the dynamic properties of the vehicle, he may not need any assistance from such control systems, and may even find such assistance superfluous and undesirable.
For instance, according to the system proposed in Japanese patent laid-open publication (kokai) No. 3-217362, the skill of the vehicle operator is evaluated according to how quickly a corrective steering action is taken when the vehicle is steered by a difference in the tractions of the right and left wheels against the will of the vehicle operator. It was proposed to permit a certain amount of difference between the tractions of the right and left wheels to exist when the level of the skill of the vehicle operator is relatively high, and control the difference in the tractions of the right and left wheels when the level of the skill of the vehicle operator is relatively low.
According to the invention disclosed in Japanese patent laid open publication (kokai) No. 4-303007, the system estimates what kind of the road, such as a city street, a highway or a country road, the vehicle is travelling from the lateral acceleration, the travelling speed of the vehicle and other parameters, and estimates the skill of the vehicle operator according to the lateral acceleration, the road condition and the steering angle. It is also estimated if the vehicle operator is in a hurry. Depending on the results of such estimation, the system selects a suitable running mode from such modes as a sport mode and a normal mode of the vehicle for the four wheel steering system or other systems of the vehicle so that the driveability of the vehicle may be improved according to such estimated data.
However, according to the proposal of Japanese patent laid-open publication (kokai) No. 3-217362, the skill of the vehicle operator can be evaluated only when the vehicle is steered by a difference in the tractions of the right and left wheels. This situation may not be present most of the time. Furthermore, because the skill of the vehicle operator normally depends on the road condition even for the same vehicle operator, this proposed system would not allow his skill to be constantly evaluated and promptly updated for each new road condition, and the vehicle control system would not be optimized most of the time.
The proposal of Japanese patent laid open publication (kokai) No. 4-303007 also has its shortcomings. Because the driving skill of the vehicle operator estimated from the lateral acceleration, the road conditions and the steering angler are used as control data in a feed-forward system, the control action at each time point is not necessarily optimized. In particular, this system is unsuitable to a situation in which the road condition constantly changes, and the vehicle often encounters unexpected disturbances.